Coelacanth is any sarcopterygianfish of the subclass Coelacanthimorpha (Actinistia) and order Coelacanthiformes, characterized by a three-lobed, diphycercal caudal fin, external nostrils, and an anterior dorsal fin in front of the center of the body (Nelson 2004). While coelacanths are well recognized in the fossil record, there are only two living species, Latimeria chalumne and Latimeria menadoensis (Nelson 2006).

The coelacanths, which are related to lungfishes and tetrapods, were believed to have been extinct since the end of the Cretaceous period, until the first Latimeria specimen (L. chalumne) was found off the east coast of South Africa in 1938. It would be 14 years before a second specimen was captured and another 45 years before another species was identified. The discovery of fish known only from the fossil record has been an exciting story of discovery, propelling an otherwise very rare fish to the forefront of popularity.

Overview and description

Coelacanths, together with lungfish, are part of the vertebrate class Sarcopterygii. The bony fish representatives of this class, known as lobe-finned fish, are characterized by lobed paired fins, which are joined to the body by a single bone (Clack 2002). Sarcopterygians also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians (ray-finned fishes). The braincase of sarcoptergygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early sarcoptergians have a symmetrical tail. Some taxonomists also include the tetrapods (amphibians, reptiles, birds, mammals) in Sacropterygii in order to make this a monophyletic group (Nelson 2006).

The coelacanths are placed in the subclass Coelacanthimorpha, while the lungfish traditionally are placed in the subclass Dipnoi, although Nelson (2006) designates Dipnotetrapodomorpha as the subclass with lungfish,

Coelacanthimorpha includes one order, Coelacanthiformes. Within Coelacanthiformes, Nelson (2006) recognizes nine families, of which eight include only fossil members and one includes two living species of coelacanth as well as fossil members. Coelacanths (order Coelacanthiformes), living and extinct, are characterized by a diphycercal caudal fin (having a tail fin with the vertebral column extending to the tip, with symmetrical lower and upper parts), and an anterior dorsal fin in front of the center of the body (Nelson 2004). Coelacanths are lobe-finned fish with the pectoral and anal fins on fleshy stalks supported by bones, and the tail or caudal fin is divided into three lobes, the middle one of which also includes a continuation of the notochord. Coelacanths have modified cosmoid scales, which are thinner than true cosmoid scales, which can only be found on extinct fish. Extant coelacanths also have a special electroreceptive device called a rostral organ in the front of the skull, which probably helps in prey detection. The small device also could help the balance of the fish, as echolocation could be a factor in the way this fish moves.

Coelacanth (pronounced/ˈsiːləkænθ/) is an adaptation of Modern Latin Cœlacanthus > cœl-us + acanth-us from Greek κοῖλ-ος [hollow] + ἄκανθ-α [spine]). This is an order of fish that includes the oldest living lineage of gnathostomata known to date.

Living species

There are two living species of coelacanths, Latimeria chalumne and L. menadoensis (Nelson 2006). These are placed in the family Latimeriidae. The first living species ever found, Latimeria chalumnae, is a marine species. It was first found off of South Africa in 1938. Since 1938, Latimeria chalumnae have been found in the Comoros, Kenya, Tanzania, Mozambique, Madagascar, and in Greater St. Lucia Wetland Park in South Africa. Adults reach 1.8 meters and it has an unusual method of swimming in keeping its body rigid (Nelson 2006). It is the only living chordate with an intracranial joint, although this feature appears in other coelacanths (Nelson 2006). It is live-bearing.

The second species, L. menadoensis, was described by Pouyaud et al. in 1999, after first being sighted in an Indonesian fish market in 1997 and the preservation of a second specimen in 1998 (Nelson 2006; Erdmann 1999). Erdmann (1999) documented these events in a 1999 issue of Environmental Biology of Fishes.

Fossil record

Coelacanths first appear in the fossil record in the Middle Devonian, about 410 million years ago. A fossil coelacanth jaw found in a stratum datable 410 mya that was collected near Buchan in Victoria, Australia's East Gippsland, currently holds the record for oldest coelacanth; it was given the name Eoactinistia foreyi when it was published in September 2006 (Burgess 2006). Prehistoric species of coelacanth lived in many bodies of water in Late Paleozoic and Mesozoic times.

Although now represented by only two known living species, as a group the coelacanths were once very successful with many genera and species that left an abundant fossil record from the Devonian to the end of the Cretaceous period, at which point they apparently suffered a nearly complete extinction. It is often claimed that the coelacanth has remained unchanged for millions of years, but, in fact, the living species and even genus are unknown from the fossil record. However, some of the extinct species, particularly those of the last known fossil coelacanth, the Cretaceous genus Macropoma, closely resemble the living species.

The average weight of the living West Indian Ocean coelacanth, Latimeria chalumnae, is 80 kilograms (176 pounds), and they can reach up to 2 meters (6.5 feet) in length. Adult females are slightly larger than males. Based on growth rings in their ear bones (otoliths), scientists infer that individual coelacanths may live as long as 80 to 100 years. Coelacanths live as deep as 700 meters (2300 feet) below sea level, but are more usually found at depths of 90 to 200 meters. Living examples of Latimeria chalumnae have a deep blue color, which probably camouflages them from prey species; however, the Indonesian species is brown. Latimeria chalumnae is widely but very sparsely distributed around the rim of the western Indian Ocean, from South Africa northward along the east African coast to Kenya, the Comoro Island and Madagascar, seemingly occurring in small colonies.

Coelacanth eyes are very sensitive, and have a tapetum lucidum. Coelacanths are almost never caught in the daytime or on nights with full moons, due to the sensitivity of their eyes. Coelacanth eyes also have many rods: receptors in the retina that help animals see in dim light. Together, the rods and tapetum help the fish see better in dark water.

Coelacanth model in the Oxford University Museum of Natural History

Coelacanths are opportunistic feeders, hunting cuttlefish, squid, snipe eels, small sharks, and other fish found in their deep reef and volcanic slope habitats. Coelacanths are also known to swim head down, backwards, or belly up to locate their prey, presumably utilizing their rostral gland. Scientists suspect that one reason this fish has been so successful is that specimens are able to slow down their metabolisms at will, sinking into the less-inhabited depths and minimizing their nutritional requirements in a sort of hibernation mode.

The coelacanths that live near Sodwana Bay, South Africa rest in caves at depths of 90 to 150 meters during daylight hours, but disperse and swim to depths as shallow as 55 meters when hunting at night. The depth is not as important as their need for very dim light and, more importantly, for water which has a temperature of 14 to 22°C. They will rise or sink to find these conditions. The amount of oxygen that their blood can absorb from the water through the gills is dependent on water temperature. Scientific research suggests that the coelacanth must stay in cold, well-oxygenated water or else their blood cannot absorb enough oxygen (Weinberg 2000).

In accordance with the Convention on International Trade of Endangered Species treaty, the coelacanth was added to Appendix I (threatened with extinction) in 1989. The treaty forbids international trade for commercial purposes and regulates all trade, including sending specimens to museums, through a system of permits. In 1998, the total coelacanth population was estimated to have been 500 or fewer, a number that would threaten the survival of the species (Jewett 1998).

Female coelacanths give birth to live young, called "pups," in groups of between 5 and 25 fry at a time; the pups are capable of surviving on their own immediately after birth. Their reproductive behaviors are not well known, but it is believed that they are not sexually mature until after 20 years of age. Gestation time is 13 months.

(July 15) Two fishermen from Zanzibar caught a coelacanth measuring 1.34 meters (4.40 feet), and weighing 27 kilograms (60 pounds). The fish was caught off the north tip of the island, off the coast of Tanzania.

First find in South Africa

Marjorie Courtenay-Latimer with the first discovered Coelacanth.

On December 23, 1938, Hendrik Goosen, the captain of the trawler Nerine, returned to the harbor at East London, South Africa, after a trawl around the mouth of the Chalumna River. As he frequently did, he telephoned his friend, Marjorie Courtenay-Latimer, curator at East London's small museum, to see if she wanted to look over the contents of the catch for anything interesting. At the harbor, Latimer noticed a blue fin and took a closer look. There she found what she later described as "the most beautiful fish I had ever seen, five feet long, and a pale mauve blue with iridescent silver markings."

Failing to find a description of the creature in any of her books, she attempted to contact her friend, Professor James Leonard Brierley Smith, but he was away for Christmas. Unable to preserve the fish, she reluctantly sent it to a taxidermist. When Smith returned, he immediately recognized it as a coelacanth, the group known previously only from fossils. Smith named the fish Latimeria chalumnae in honor of Marjorie Courtenay-Latimer and the waters in which it was found. The two discoverers received immediate recognition, and the fish became known as a "living fossil." The 1938 coelacanth is still on display in the East London, South Africa, museum.

However, as the specimen had been stuffed, the gills and skeleton were not available for examination, and some doubt therefore remained as to its identity. Smith began a hunt for a second specimen that would take more than a decade.

Comoros

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A worldwide search was launched for more coelacanths, with a reward of 100 British pounds, a very substantial sum to the average South African fisherman of the time. Fourteen years later, one specimen was found in the Comoros, but the fish was no stranger to the locals—in the port of Mutsamudu on the Comorian island of Anjouan, the Comorians were puzzled to be so rewarded for a "gombessa" or "mame," their names for the nearly inedible fish that their fishermen occasionally caught by mistake.

The second specimen, found in 1952 by Comorian fisherman Ahamadi Abdallah, was described as a different species, first as Malania hunti and later as Malania anjounae, after Daniel François Malan, the South African Prime Minister who had dispatched an SAAF Dakota at the behest of Professor Smith to fetch the specimen. It was later discovered that the lack of a first dorsal fin, at first thought to be significant, was caused by an injury early in the specimen's life. Ironically, Malan was a staunch creationist; when he was first shown the primitive creature, he exclaimed, with a twinkle, "My, it is ugly. Do you mean to say we once looked like that?" (Weinberg 2000). The specimen retrieved by Smith is on display at the SAIAB in Grahamstown, South Africa where he worked.

The Comorians are now aware of the significance of the endangered species and have established a program to return any accidentally-caught coelacanth to deep water.

As for Smith, who died in 1968, his account of the coelacanth story appeared in the book Old Fourlegs, first published in 1956. His book Sea Fishes of the Indian Ocean, illustrated and co-authored by his wife Margaret, remains the standard ichthyological reference for the region.

In 1988, National Geographic photographer Hans Fricke was the first to photograph the species in its natural habitat, 180 meters (590 feet) off Grande Comore's west coast (Fricke 1988).

Second species in Indonesia

On September 18, 1997, Arnaz and Mark Erdmann, traveling in Indonesia on their honeymoon, saw a strange fish enter the market at Manado Tua, on the island of Sulawesi (Jewett 1998). Mark thought it was a gombessa (Comoros coelacanth), although it was brown, not blue. An expert noticed their pictures on the Internet and realized its significance. Subsequently, the Erdmanns contacted local fishermen and asked for any future catches of the fish to be brought to them. A second Indonesian specimen, 1.2 meters in length and weighing 29 kilograms was captured alive on July 30, 1998 (Nelson 2006). It lived for six hours, allowing scientists to photographically document its coloration, fin movements, and general behavior. The specimen was preserved and donated to the Museum Zoologicum Bogoriense (MZB), part of the Indonesian Institute of Sciences (LIPI) (Jewett 1998).

DNA testing revealed that this specimen differed genetically from the Comorian population. Superficially, the Indonesian coelacanth, locally called raja laut ("King of the Sea"), appears to be the same as those found in the Comoros except that the background coloration of the skin is brownish-gray rather than bluish. This fish was described in by Pouyard et al. in 1999. It was given the scientific nameLatimeria menadoensis. A molecular study estimated the divergence time between the two coelacanth species to be 40 to 30 million years ago (Inoue et al. 2005).

On May 19, 2007, Justinus Lahama, an Indonesian fisherman, caught a 1.3-meter-long, 50-kilogram (110-pound) coelacanth off the coast near Manado, on northern Sulawesi Island near Bunaken National Marine Park. After spending 30 minutes out of water, the fish, still alive, was placed in a netted pool in front of a restaurant at the edge of the sea. It survived for 17 hours. Coelacanths, closely related to lungfish, usually live at depths of 200 to 1,000 meters. The fish was filmed by local authorities swimming in the meter-deep pool, then frozen after it died. Scientists working with the French Institute for Development and Research carried out a necropsy on the coelacanth with genetic analysis to follow.

St. Lucia Marine Protected Area in South Africa

In South Africa, the search continued on and off over the years. On October 28, 2000, just south of the Mozambique border in Sodwana Bay in the St. Lucia Marine Protected Area, three deep-water divers, Pieter Venter, Peter Timm, and Etienne le Roux, made a dive to 104 meters and unexpectedly spotted a coelacanth.

Calling themselves "SA Coelacanth Expedition 2000," the group returned with photographic equipment and several additional members. On the 27th of November, after an unsuccessful initial dive the previous day, four members of the group, Pieter Venter, Gilbert Gunn, Christo Serfontein, and Dennis Harding, found three coelacanths. The largest was between 1.5 and 1.8 meters in length; the other two were from 1 to 1.2 meters. The fish swam head-down and appeared to be feeding from the cavern ledges. The group returned with video footage and photographs of the coelacanths.

During the dive, however, Serfontein lost consciousness, and 34-year-old Dennis Harding rose to the surface with him in an uncontrolled ascent. Harding complained of neck pains and died from a cerebral embolism while on the boat. Serfontein recovered after being taken underwater for decompression sickness treatment.

In March–April of 2002, the Jago Submersible and Fricke Dive Team descended into the depths off Sodwana and observed fifteen coelacanths. A dart probe was used to collect tissue samples.

The shallowest recorded sighting of a coelacanth is at a depth of 58 meters off the coast of Sodwana Bay by Christo Vanjaarsveld.

Tanzania

Coelacanths have been caught off the coast of Tanzania since 2004. Two coelacanths were initially reported captured in Kigombe, a small village off the edge of the Indian Ocean in August 2004. A spate of 19 more specimens of these extremely rare fishes, weighing between 25 kilograms to 80 kilograms, were reported netted in the space of the next 5 months, with another specimen captured in January 2005. A coelacanth weighing as much as 110 kilograms was reported by the Observer newspaper in 2006. Officials of the Tanga Coastal Zone Conservation and Development Programme, which has a long-term strategy for protecting the species, see a connection with the timing of the captures with trawling—especially by Japanese vessels—near the coelacanth's habitat, as within a couple of days of trawlers casting their nets coelacanths have turned up in shallow-water fishing nets intended for sharks. The sudden appearance of the coelacanth off Tanzania has raised real worries about its future due to damage done to the coelacanth population by the effects of indiscriminate trawling methods and habitat damage (Gilmore 2006).

Descendants also included pelagic lobe-finned fish such as coelacanth species.

Subclass Coelacanthimorpha (Actinistia) are sometimes used to designate the group of Sarcopterygian fish that contains the Coelacanthiformes. The following is a classification of known coelacanth genera and families (Nelson 2006):

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